1. Field of the Invention
The present invention relates to an optical amplification fiber for amplifying a signal, a fiber optic amplifier including the optical amplification fiber, and an optical communication system including the fiber optic amplifier.
2. Related Background Art
Along with recent proliferation of the Internet, demands for communication capacities are rapidly increasing. In particular, a transoceanic optical communication system that connects continents is mainly used for international communication which requires a large capacity, and its communication distance is as long as several thousand kilometers. In such a long-haul optical communication system, a fiber optic amplifier is indispensable. A fiber optic amplifier includes an optical amplification fiber having a light propagating region doped with a rare earth element. When pumping light is supplied to the optical amplification fiber, signals can be amplified. For example, generally, the rare earth element is Er, the pumping light wavelength is 0.98- or 1.48-xcexcm band, and the signal wavelength is a 1.55-xcexcm band.
The present inventors examined the above-described prior art and found the following problem. There are many technical difficulties for a fiber optic amplifier to sufficiently exhibit its performance in a long-haul optical communication system. That is, an increase in output power of amplified signals simultaneously causes an increase in noise figure (NF), through the noise figure of the entire transmission medium such as an optical amplification fiber through which light in the signal wavelength band and light in a pumping light wavelength band different from the signal wavelength band simultaneously propagate cannot be predicted. The conventional fiber optic amplifier does not sufficiently exhibit its amplification performance and, more particularly, performance related to the noise figure NF, and therefore, the performance of the optical communication system is not sufficient, either.
The present invention has been made to solve the above-described problem, and has as its object to provide an optical amplification fiber having a structure capable of exhibiting an excellent amplification characteristic, a fiber optic amplifier including the optical amplification fiber, and an optical communication system including the fiber optic amplifier.
An optical amplification fiber according to the present invention is an optical amplification fiber having a light propagating region whose at least part is doped with a rare earth element, and being capable of amplifying signals by using supplied pumping light. The optical amplification fiber has a structure for reducing the noise figure NF by unbalancing a mode field diameter for pumping light having a wavelength of 0.98 xcexcm (975 to 985 nm) and a mode field diameter for signals having a wavelength of 1.55 xcexcm (1,525 to 1,565 nm) and also has a structure for suppressing the noise figure NF to an appropriate value in a transmission medium through which these light components in different wavelength bands simultaneously propagate. That is, in the optical amplification fiber according to the present invention, the ratio (xcex1P/xcex1S) of an unsaturated absorption peak value xcex1P in a pumping light wavelength band of 0.98 xcexcm to an unsaturated absorption peak value xcex1S in a signal wavelength band of 1.55 xcexcm is controlled to be 0.8 or more and, more preferably, 0.9 or more. In the optical amplification fiber according to the present invention, the rare earth element is added into the light propagating region of the optical amplification fiber such that its concentration increases toward the optical axis of the optical amplification fiber.
A fiber optic amplifier according to the present invention comprises an optical amplification fiber having the above-described structure (an optical amplification fiber according to the present invention), and a pumping light source for supplying the pumping light to the optical amplification fiber.
An optical communication system according to the present invention is a WDM (Wavelength Division Multiplexing) system having a transmission medium for propagating multiplexed signals of a plurality of channels with different wavelengths, which includes the above fiber optic amplifier (a fiber optic amplifier according to the present invention) capable of amplifying these WDM signals.
In the fiber optic amplifier including the optical amplification fiber with the above-described structure, the noise figure NF is suppressed to 0.4 dB or less, and an excellent amplification characteristic can be exhibited. In the optical communication system including the fiber optic amplifier with the above-described structure, WDM signals can be transmitted through a long haul at a high bit rate without any reception error.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.